From the back cover:
Cthulhu and his minions plot against the peace and surety of the natural world, but so do a host of Powers distinct from him. Along with Great Cthulhu, these entities comprise the Great Old Ones, awesome beings who came to this planet hundreds of millions of years ago, and who dwell here yet in the dark places, just beyond the reach of man.
THE GREAT OLD ONES consists of a set of six scenarios for Call of Cthulhu: "The Spawn" is in the Wild West, with Indians, Wobblies, and bad guys; "Still Waters" is an adventure for people who hate to lend books; "Tell Me, Have You Seen the Yellow Sign?" makes a symbolic stop-over in New Orleans; "One in Darkness" features South Boston hoodlums; "The Pale God" introduces investigators to an unusual contract; "Bad Moon Rising" is an experience to remember. The adventures can be presented in sequence, as a loose campaign; limited cross-references allow the scenarios to stand independently.
This book is a companion to publications such as CTHULHU NOW, H.P. LOVECRAFT'S DREAMLANDS, CTHULHU BY GASLIGHT, and TERROR AUSTRALIS, which situate Cthulhu players in various times and places.